In a typical digital transmission system, a digital signal is transmitted along a transmission path such as a pair of wires in a telephone cable having digital repeaters periodically interposed which regenerate the signal. Due to the proximity of the wires in the cable, crosstalk interference is generated of which the three most significant types are: near-end crosstalk (NEXT), far-end crosstalk (FEXT) and near-end-near-end interaction crosstalk (NENEIXT). Of these three the most disturbing is generally NEXT which results when high-level regenerated signals are coupled directly into the paths carrying low-level signals in the opposite direction. An interesting discussion on crosstalk interference can be found in a paper entitled: "Engineering T1C Carrier Systems" by J. P. Fitzimmons and W. J. Mayback, Conference Record, ICC 75, June 16-18 San Francisco, Vol.III, pp. 39-5 to 39-9.
In many applications, binary data is transmitted over the cable pairs using ternary partial-response signals such as bipolar, duobinary and modified duobinary signals. References to such systems are "Correlative Level Coding For Binary-Data Transmission" by Adam Lender, IEEE Spectrum, February 1966, pp. 104-115; "Transmission Systems For Communications" 4th Edition by Bell Telephone Laboratories, Inc., pp. 666-673; and "Principles of Data Communication" by R. W. Lucky, J. Salz and E. J. Weldon Jr., McGraw-Hill 1968, pp. 83-92.
The basic form of a digital repeater used to regenerate such a transmitted signal is discussed in an article entitled: "A New 3.152Mb/s Digital Repeater" by A. Anuff et al, Conference Record, ICC 75, June 16-18 San Franciso, Vol.3,pp. 39-10 to 39-13. In such a repeater, the signal is first passed through an equalizer which compensates for the transfer characteristics of the telephone cable. A regenerator then reconstructs the signal in its original form for transmission along the following section of the transmission line.
In many systems, the interfering power density of the noise varies with frequency. For instance in cable circuits, it is well known that NEXT is frequency dependent and usually increases with frequency at 4.5dB/octave rates; ibid BTL text, pp. 286-288. To minimize the NEXT effect, the bandwidth of the equalizer should be as narrow as possible. However, the response of the equalizer has in the past been such as to minimize intersymbol interference thereby restricting its minimum bandwidth. This has frequently led to the use of a raised-cosine transmission characteristic; ibid BTL text, pp. 715-718. In other systems, the power density of the interfering noise may decrease with frequency such as when it is coupled from an adjacent power line. In still other systems, the noise appears only at the band edges. An example of this is the noise which results from inter-channel interference in a digital radio system.